Editor's note: Trend Charts and rankings for metropolitan areas and county grades are available at stateoftheair.org

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The American Lung Association's 2016 "State of the Air" report found continued improvement in air quality, but more than half (52.1%) of the people in the United States live in counties that have unhealthful levels of either ozone or particle pollution. The annual, national air quality "report card" found that 166 million Americans live with unhealthful levels of air pollution, putting them at risk for premature death and other serious health effects like lung cancer, asthma attacks, cardiovascular damage, and developmental and reproductive harm.

"Thanks to cleaner power plants and cleaner vehicles, we see a continued reduction of ozone and year-round particle pollution in the 2016 'State of the Air' report. However, climate change has increased the challenges to protecting public health," said Harold P. Wimmer, National President and CEO of the American Lung Association. "There are still nearly 20 million people in the United States that live with unhealthful levels of all three measures of air pollution the report tracks: ozone, short-term and year-round particle pollution."

"Everyone has the right to breathe healthy air, yet only four cities—Burlington-South Burlington, Vt.; Elmira-Corning, N.Y.; Honolulu, Hawaii; and Salinas, Calif.—qualified for the 'cleanest cities' list in the "State of the Air' report," Wimmer said. "We simply must do more to protect the health of Americans."

Each year the "State of the Air" reports on the two most widespread outdoor air pollutants, ozone pollution and particle pollution. The report analyzes particle pollution in two ways: through average annual particle pollution levels and short-term spikes in particle pollution. Both ozone and particle pollution are dangerous to public health and can be lethal. But the trends reported in this year's report, which covers data collected in 2012-2014, are strikingly different for these pollutants.

According to this year's 17th annual report, short-term spikes in particle pollution have gotten worse since the 2015 report, including in the city with the worst particle pollution problem, Bakersfield, Calif. For multiple cities that suffered spikes in particle pollution during this period, many of these spikes were directly linked to weather patterns like drought or to events like wildfires, which are likely to increase because of climate change.

Top 10 U.S. Cities Most Polluted by Short-Term Particle Pollution (24-hour PM2.5):

Bakersfield, Calif.

Fresno-Madera, Calif.

Visalia-Porterville-Hanford, Calif.

Modesto-Merced, Calif.

Fairbanks, Ala.

Salt lake City-Provo-Orem, Utah

Logan, Utah-Idaho

San Jose-San Francisco-Oakland, Calif.

Los Angeles-Long Beach, Calif.

Missoula, Mont.

The rise in short-term particle pollution provides current examples of how major changes in drought and rainfall are already affecting public health. According to the 2016 report, seven of the 25 most polluted cities had their highest number of unhealthy short-term particle pollution days on average ever reported.

Increased heat, changes in climate patterns, drought and wild fires are all related to climate change, which has contributed to the extraordinarily high numbers of days with unhealthy particulate matter. The particles—emanating from wildfires, woodburning devices, coal-fired power plants and diesel emissions—are so small that they can lodge deep in the lungs and trigger asthma attacks, heart attacks and strokes, and can even be lethal.

By contrast, the best progress came in reducing year-round levels of particle pollution, with 16 cities reaching their lowest levels ever, and one other improving over the period covered by the 2015 report (2011-2013). Year-round particle pollution levels have dropped thanks to the cleanup of coal-fired power plants and the retirement of old, dirty diesel engines.

Top 10 U.S. Cities Most Polluted by Year-Round Particle Pollution (Annual PM2.5):

Bakersfield, Calif.

Visalia-Porterville-Hanford, Calif.

Fresno-Madera, Calif.

Los Angeles-Long Beach, Calif.

El Centro, Calif.

(tied) Modesto-Merced, Calif.

(tied) San Jose-San Francisco-Oakland, Calif.

Pittsburgh-New Castle-Weirton, Pa.-Ohio-W. Va.

Harrisburg-York-Lebanon, Pa.

Louisville-Jefferson

County-Elizabethtown-Madison, KY-IN

According to the 2016 report, six cities reported their fewest unhealthy ozone days ever, including #1 Los Angeles, and 15 others improved over the previous report’s data. Ozone pollution has decreased because the nation has cleaned up major sources of the emissions that create ozone, especially coal-fired power plants and vehicles. However, climate change causes greater heat, which makes ozone form. When a person inhales ozone pollution, it can cause coughing, trigger asthma attacks, and even shorten life.

Top 10 Most Ozone-Polluted Cities:

Los Angeles-Long Beach, Calif.

Bakersfield, Calif.

Visalia-Porterville-Hanford, Calif.

Fresno-Madera, Calif.

Phoenix-Mesa-Scottsdale, Ariz.

Sacramento-Roseville, Calif.

Modesto-Merced, Calif.

Denver-Aurora, Colo.

Las Vegas-Henderson, Nev.-Ariz.

Fort Collins, Colo.

"We can and must do more to save lives and fight climate change," Wimmer said. "The Lung Association calls on every state to adopt strong Clean Power Plans to reduce emissions from power plants that worsen climate change and immediately harm health. The Supreme Court has put a temporary hold on EPA’s enforcement of the federal Clean Power Plan, but states should not wait to clean up carbon pollution from their power plants."

"In addition, we call on EPA to adopt strong, health protective standards to limit emissions of methane and toxic pollutants that contribute to ozone pollution and climate change from oil and gas production," he said.

Learn more about the 2016 "State of the Air" report at www.stateoftheair.org. For media interested in speaking with an expert about lung health and healthy air, contact the American Lung Association at [email protected] or 312-801-7628.